There is a certain ratio of weight to horsepower to job that is ideal. Saying a heavier or lighter tractor will give you better traction and is therefore a better tractor is overly simplistic. One must keep in mind which wheels are driven and how the weight is distributed and what job you are trying to perform. A MFWD tractor will need the weight displaced differently then a 2wd model. A tractor that is mowing will not need the weight of a tractor that is plowing. Ballasting a tractor is more often a good thing, not a bad thing.
Here is link with good info:
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Here is some info that has been shamelessly lifted from the Ropin' Web (I'm giving credit where credit is due). There is also some good advice on tire pressures, etc.
Ballasting for efficiency
Most farmers reach the upper limits of their tractor's power only 15 to 20 percent of the time. With this in mind, weighting the tractor for typical conditions rather than maximum needs will reduce fuel costs.
Farmers usually look at ballasting from a worst-case scenario, says Turner. "If they need to pull 20,000-pound loads for two weeks of the year, they
ballast their tractors for that load 52 weeks a year. They might be better off to ballast their tractors for 15,000-pound loads. While that means they could struggle a bit for those two weeks a year, it's more efficient than carrying the weight for the other 50 weeks," he says.
Ideally, farmers should ballast their tractors for specific loads, but it can be a hassle, says Turner. "The key to ballasting is to decide the speed necessary for an operation and the amount of the load. Then set the tractor weight just heavy enough to pull that load at that speed and at an acceptable slip level. The total tractor weight should wind up being 2.5 to 3 times the load being pulled."
When a tractor is over-ballasted, excessive torque can be transmitted through the drivetrain and tires to the ground, he says. That can cause overloads, wear and drivetrain failures. As well, fuel costs are increased from carrying the extra weight and from the inefficient power transfer.
"Over-ballasting is a common farmer error," says Turner. "An over-ballasted tractor will probably feel sluggish, and on top of burning more fuel than it should, may experience premature drivetrain problems."
An under-ballasted tractor wears tire tread at a faster pace because of excessive slip, while never delivering full horsepower to the drawbar, he says. Fuel is wasted because of the extra wheel revolutions to travel the same distance.
When fine-tuning ballast distribution, it's important to consider wheel slip, horsepower and speed. The total ballasted weight for drawn implements should balance as follows:
Front Back
2WD 25% 75%
FWD 40% 60%
4WD 55% 45%
For further information, see Research Update 725, from the AgTech Centre.